Optimization on proportion for recycled aggregate in concrete using two-stage mixing approach

Recycled aggregate (RA) is well acknowledged having a poorer quality due to its higher porosity resulted from cement mortar remains attaching to its surface that hampers the recycling rate of concrete waste. Many previous researches recorded reduction in strength for concrete made with RA. As a result, the use of RA is mainly confined to low-grade applications. Tam et al. [Tam WYV, Gao XF, Tam CM. Micro-structural analysis of recycled aggregate concrete produced from two-stage mixing approach. Cem Concr Res 2005;35(6):1195-203] proposed a two-stage mixing approach (TSMA) for improving the strength of recycled aggregate concrete (RAC), by testing mixes with up to 30% RA replacement. This paper extends Tam et al.'s work (2005) by exploring RA substitutions ranging from 0% to 100% and compares their performance with the traditional mixing procedure. Based upon the experimental works, improvements on strength and rigidity of RAC using TSMA were compared with those of traditional mixing procedure based on different percentages of RA replacements. The results were then optimized using general regression neural networks (GRNN) and RA replacements of 25-40% and 50-70% were found to be optimal when TSMA was adopted. It confirms the conservative recommendation of 20% RA substitution by many previous researchers and public users.